In communication systems (e.g., including both terminal and base station infrastructure), surface acoustic wave (SAW) devices, such as filters and resonators, are widely used. A SAW device may be implemented using an interdigital transducer (IDT). Typically, a SAW generated by the IDT is bidirectional. In the case of a transversal SAW filter, the SAW device has an input IDT and an output IDT. A SAW generated by the input IDT propagates bidirectionally, such that only 50% of the generated energy travels in the direction of the output IDT, with the remaining 50% being propagated away from the output IDT. Thus, the SAW filter suffers about 6 dB loss due to the bidirectionality of its transducers, with about 3 dB losses at each IDT. One way to reduce the insertion loss of a transversal SAW filter is to design the SAW filter such that a SAW generated by the input transducer propagates unidirectionally. Similarly, the output transducer should also be designed unidirectionally (in a direction opposite to the input transducer) to receive the SAW.
Single-phase unidirectional transducers (SPUDTs) have been designed to achieve this unidirectional wave propagation. Examples of SPUDTs include natural SPUDTs (NSPUDTs), floating electrode unidirectional transducers (FEUDTs), and film thickness difference type SPUDTs, among others. However, such designs have drawbacks. For example, devices using NSPUDTs rely on asymmetry of the crystal substrate (i.e., difference in acoustic wave propagation properties between the forward direction and the reverse direction) to achieve unidirectional wave propagation at the input transducer, but because the output transducer is on the same crystal substrate there is no reversal of the wave direction at the output transducer. In devices using FEUDTs, the transducer design requires narrow electrodes that are designed to have a width that corresponds to a small fraction of the operating wavelength (e.g., 1/12 of the operating wavelength). Because the smallest width of the electrodes is limited by the minimum feature size possible under practical fabrication considerations (typically about 0.2 μm), the result is that a very high operating frequency (i.e., a very short operating wavelength) may not be achievable. For devices using film thickness difference type SPUDTs, the electrode design is complicated and typically leads to difficulty in fabrication.
It would be desirable to provide a SAW device with a unidirectional transducer, without relying on asymmetry of the crystal substrate, and with a relatively straightforward fabrication process, yet achieving a very high operating frequency.